ASoC: sgtl5000: enable VAG_POWER for LINE_IN

LINE_IN also needs VAG_POWER on or we may hear noise when directly
route LINE_IN to Headphone Mux.
Tested on imx28evk.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>